1. Field of the Invention
The present invention relates to a scanning probe microscope that is a general term indicating various instruments, including scanning tunneling microscope, atomic force microscope, magnetic force microscope, frictional force microscope, micro-viscoelasticity AFM, and scanning Kelvin probe microscope (SKPM).
2. Description of Related Art
A scanning probe microscope uses a probe that is brought close to a surface of a sample made of a metal, semiconductor, insulator, or high-molecular weight material or consisting of a biological sample. The microscope scans the surface of the sample such that the tunneling current, interatomic force, or the like produced between the probe and the sample surface is maintained constant. Thus, the surface topography and physical properties are measured at the atomic level.
In the non-contact mode of atomic force microscopy, observations and measurements are performed using the interatomic attraction force between the sample surface and probe. Since the interatomic attraction force is detected from the non-contact distance and imaging is done, damage to the sample is small. Hence, the top layer of the surface of the sample can be imaged accurately. The microscope is equipped with a piezo element for vibrating the cantilever in contact with the base portion of the cantilever. The cantilever has a natural frequency of tens to hundreds of kilohertz depending on the length and thickness of the cantilever. When a driving signal of the natural frequency of oscillation is applied to the piezo element for applying vibrations from an oscillator, the free end forming the probe is vibrated up and down at an amplitude on the order of nanometers at the natural vibration frequency f0. Where this state is taken as a steady state and the probe is brought close to the sample, an interatomic force acts between the probe and sample at the lowest point. When the probe receives the interatomic force, the frequency shifts downward compared with the vibrational frequency f0 in the steady state. That is, the vibrational period becomes elongated.
In the non-contact mode of AFM using an optical lever technique, laser light reflected from the cantilever contains various frequency components, including components attributed to frequency variation Δf, amplitude variation ΔA, and phase variation due to the action of the attraction force. Using any one of these variations as a feedback signal, the probe-sample distance is kept constant. In detecting the feedback signal, the slope detection method in which the frequency shift is indirectly detected within the atmosphere is generally used. In a vacuum, the FM detection method in which the shift is directly detected is generally used. The prior art scanning probe microscope is shown in the block diagram of FIG. 1.
Where observations and measurements are performed using a scanning probe microscope, even if probes are fabricated at the highest accuracy, good observational results may not be obtained because of atomic-level observation and because of variations in individual probes. Furthermore, during observation, foreign material may adhere to the probe, deteriorating the sensitivity.
In this case, it has been empirically known that an improvement may be achieved by increasing the amplitude of the vibrating probe and bringing the probe into contact with the sample. It is considered that the contact has deformed the state of the tip of the probe at the atomic level, creating a state adapted for observation of the atomic image. It is also considered that the adhering material has been cleaned off.
Where the prior art control computer is used and the scroll bar on the viewing screen of the computer is dragged with the mouse to change the set value of the amplitude of the probe, the scroll bar has been varied suddenly with the mouse. The set value is modified to a value at which a contact might be made or to a value at which the amplitude is slightly smaller. Then, the probe is brought into contact with the sample. This method is not a quantitative technique and so the probe may be overloaded.
Where the numerical value of the set value of the probe amplitude is entered, a value at which the probe makes contact with the sample has been entered from the keyboard of the control computer. In this case of entry of numerical value, quantitative setting is possible. However, the operation is slower than the entry from the scroll bar. If the operation is made slower, the scanning probe may be overloaded.
As a known technique, a cleaning device for the probe of a scanning probe microscope is disclosed in Japanese Patent Laid-Open No. H8-226927.